Protein phosphorylation regulates many processes in animal cells. Protein kinases and phosphatases coordinately control the phosphorylation state of cellular proteins. Mechanisms that can temporarily dampen counteracting phosphatases can further amplify hormonal signals. We have investigated phosphatase inhibitor-1 (I-1) as a prototype mechanism for direct communication between a protein kinase and a phosphatase. Increases in cAMP levels activate PKA and promote phosphorylation and activation of I-1. This results in the inhibition of a major cellular phosphatase, PP1. Coordination of PKA and PP1 functions by I-1 greatly amplifies the cAMP signal, as demonstrated by expression of activated I-1 in cells which increases their sensitivity to cAMP and prolongs their physiological response. However, the molecular basis for I-1 s function as a PP1 regulator remains poorly understood. The investigators will undertake detailed structure-activity analysis of recombinant human I-1 to define its mode of action. A specific goal of the investigators' studies is to generate forms of I-1 and PP1 with altered regulatory properties. Expression of mutant and wild-type proteins in cells will further define I-1's role in cell signaling. Environmental toxins that inhibit the major cellular phosphatases, including PP1, have not only emphasized the physiological importance of endogenous inhibitors like I-1 but have raised the possibility that I-1 deregulation may contribute to human disease. Defining the functional interactions between I-1 and PP1 will lead to the design of dominate negative reagents that will be used not only to elucidate I-1 s role in normal physiology but reverse deleterious effects on I-1 overexpression. These studies are relevant to targeting phosphatases to modulate hormonal responses and ameliorate metabolic dysfunctions in many human diseases.